Multilevel data compression using a single compression engine

ABSTRACT

A single compression engine transmits first and second discrete cosine transform (DCT)-encoded signals. The first DCT-encoded signal uses at most t coefficient bits to represent each of a plurality of DCT coefficients. The second DCT-encoded signal uses at most u coefficient bits, where u is less than t, to represent each of the plurality of DCT coefficients.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent application No.10/037,453, filed Dec. 20, 2001, the entirety of which is herebyincorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to compression methods and systems.

2. Description of the Related Art

Mathematical transformations are used in compression systems torepresent audio data and/or picture data in a more efficient manner. Awidely-used mathematical transformation is the Discrete Cosine Transform(DCT). To provide a substantially-identical representation of a 270 Mb/sCCIR (Comite Consultatif International des Radiocommunications) 601video stream, a calculation accuracy of about 13 bits to 14 bits isrequired (see Robin et al., Digital Television Fundamentals,McGraw-Hill, pg. 360). Compression of the video stream may be achievedby discarding lesser significant bits or using fewer bits. Thus, thelevel of compression is directly tied to the number of places ofaccuracy maintained throughout the process of calculating the DCT.

Previous approaches to compressing a DCT representation includeallocating unequal numbers of bits for the transform coefficients. Thenumber of bits allocated to a transform coefficient may be based upon avariance of the transform coefficient. In this way, more bits areallocated to widely-varying coefficients than for lesser-varyingcoefficients.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is pointed out with particularity in the appendedclaims. However, other features are described in the following detaileddescription in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of an embodiment of a system for providing aplurality of compression levels using a single compression engine; and

FIG. 2 is a flow chart of an embodiment of a method of providing aplurality of compression levels using a single compression engine.

DETAILED DESCRIPTION OF THE DRAWINGS

Disclosed herein are a method and a system which provide a plurality oflevels of compression using a single compression engine. The singlecompression engine produces a plurality of encoded versions of a set ofdata, such as audio data and/or video data. Each encoded version has itsown data rate. This enables data links of various bandwidths to use thesame compression engine at various data rates and quality levels. Forexample, both a 6 Mb/s data link and a 3 Mb/s data link could begenerated using the same audio and/or video compression engine.

Embodiments of the present invention are described with reference toFIG. 1, which is a block diagram of an embodiment of a system forproviding a plurality of compression levels using a single compressionengine 10, and FIG. 2, which is a flow chart of an embodiment of amethod of providing a plurality of compression levels using the singlecompression engine 10. The compression engine 10 may be embodied by ageneral purpose computer system to perform the acts described herein.The computer system may be directed by computer-readable program codestored by a computer-readable medium. Alternatively, the compressionengine 10 may be embodied by an application-specific device.

As indicated by block 12, the method comprises determining a DCT of ablock of data 14. For purposes of illustration and example, the block ofdata 14 comprises an 8-by-8 block of pixels represented by x(i,j), whereindex values i and j range from 0 to 7. It is noted, however, that theteachings herein also are applicable to blocks of data having widthsother than 8-by-8, to data which may not represent video information,and to blocks of data having one dimension or more than two dimensions.A two-dimensional DCT of the block of data can be found using thefollowing formula:${Z\left( {k,l} \right)} = {\frac{1}{4}C_{K}C_{l}{\sum\limits_{i = 0}^{7}{\sum\limits_{j = 0}^{7}{{x\left( {i,j} \right)}\quad\cos\quad\frac{\pi\left( {{2i} + 1} \right)k}{16}\cos\quad\frac{\pi\left( {{2j} + 1} \right)l}{16}}}}}$where Z(k,l) represents the DCT coefficient for index values of k and l,C_(k) and C_(l), are index-dependent constants, and the index values ofk and l range from 0 to 7.

As indicated by block 16, the method comprises representing each DCTcoefficient by a corresponding series of t coefficient bits, where t isan integer greater than or equal to zero. The t coefficient bits for aDCT coefficient are represented by a₁, a₂, . . . , a_(t). Thus, a DCTcoefficient Z is related to its corresponding t coefficient bits asfollows: $Z = {\sum\limits_{m = 1}^{t}{a_{m}2^{n - 1 - m}}}$where n is a constant selected such that 2^(n) provides a suitablemost-significant bit value.

An initial calculation of the DCT coefficients may be performed witht=13 or t=14 so that 13 or 14 bits are used to represent each DCTcoefficient. This number is typically set by either a fixed number or anupper bound.

The acts indicated by blocks 12 and 16 can be repeated for other blocksof data 20, e.g. other blocks of pixels in a picture, and/or otherblocks of pixels in other pictures in a video sequence.

As indicated by block 22, the method comprises providing a firstDCT-encoded signal 24 which uses at most t coefficient bits to representeach DCT coefficient. The first DCT-encoded signal 24 may comprise a bitstream or other form of signal which encodes the at most t coefficientbits to represent each DCT coefficient. Optionally, each DCT coefficientis represented by t coefficient bits.

As indicated by block 26, the method comprises providing a secondDCT-encoded signal 30 which uses fewer than t coefficient bits torepresent each DCT coefficient. The maximum number of coefficient bitsto represent each DCT coefficient in the second DCT-encoded signal 30 isdenoted by u. The number of bits/coefficient may be reduced by removingat least one lesser-significant bit from each of the DCT coefficientsrepresented by t coefficient bits. Alternatively, the number ofbits/coefficient may be reduced by removing at least onelesser-significant bit from each of the DCT coefficients. The secondDCT-encoded signal 30 may comprise a bit stream or other form of signalwhich encodes the at most u coefficient bits to represent each DCTcoefficient.

Optionally, one or more additional DCT-encoded signals are providedwhich use fewer than t coefficient bits to represent each DCTcoefficient. For example, as indicated by block 32, the method maycomprise providing a third DCT-encoded signal 34 which uses fewer than ucoefficient bits to represent each DCT coefficient. The maximum numberof coefficient bits to represent each DCT coefficient in the thirdDCT-encoded signal 34 is denoted by v. The number of bits/coefficientmay be reduced by removing at least two lesser-significant bits fromeach of the DCT coefficients represented by t coefficient bits.Alternatively, the number of bits/coefficient may be reduced by removingat least two lesser-significant bits from each of the DCT coefficients.

Each DCT-encoded signal may be provided to a corresponding datacommunication link. For example, the first DCT-encoded signal 24 isprovided to a first data communication link 36 having a first bandwidthA, the second DCT-encoded signal 30 is provided to a second datacommunication link 40 having a second bandwidth B, and the thirdDCT-encoded signal 34 is provided to a third data communication link 42having a third bandwidth C. The first bandwidth A is greater than thesecond bandwidth B, and the second bandwidth B is greater than the thirdbandwidth C.

Preferably, the DCT-encoded signals 24, 30 and 34 are concurrentlycommunicated via the data communication links 36, 40 and 42,respectively. Optionally, the DCT-encoded signals 24, 30 and 34 aresubstantially synchronized to provide a substantially similar (otherthan the level of compression and the data rate) broadcast to recipientsthereof.

As stated above, a greater number of coefficient bits produces lesscompression. By selecting the number of coefficient bits to be sent, alevel of compression can be determined. For example, a networkdistribution center may need the full bandwidth provided by the firstDCT-encoded signal 24 which results from an upper bound of coefficientbits of 13 or 14. The network distribution center communicates the firstDCT-encoded signal 24 via the data communication link 36 having a highbandwidth. The compression engine 10 may further supply separate datastreams having fewer coefficient bits in order to fit the streams indata links of different capacity. For example, one user may choose thedata communication link 42 having a very low bit rate to communicate thethird DCT-encoded signal 34. Another user may have access to the datacommunication link 40 having a higher bandwidth, thus enabling a higherquality bit rate version such as the second DCT-encoded signal 30 to becommunicated. In general, each customer can individually choose thecompression level that fits its technical and/or economic situation.

The DCT-encoded signals 24, 30 and 34 are received and decoded byreceiver/decoders 44, 46 and 50, respectively. The present applicationcontemplates each of the receiver/decoders 44, 46 and 50 comprising adecompression engine capable of decompressing DCT-encoded signals atdifferent numbers of bits/coefficient and multiple transmission rates.Use of compression-decompression (codecs) devices capable of working atmultiple transmission rates using the same algorithm eliminates multiplecodecs for multiple services, which reduces both cost and networkcomplexity.

As video networks move from specialized overlay networks to more generaldata networks, issues such as maintaining end-user-to-end-usercompatibility will remain of interest from both a legacy compatibilityperspective and an economic perspective. It is believed that thetechniques described herein will aid in the transition by providing asingle compression method that can deliver multiple levels ofcompression over data networks.

It will be apparent to those skilled in the art that the disclosedinventions may be modified in numerous ways and may assume manyembodiments other than the preferred forms specifically set out anddescribed herein. For example, the teachings herein can be extended toother transforms used in transform coding. Further, the herein-describedbase-2 representation of the coefficients may be modified to other basevalues, e.g. base-10 or base-16. In these cases, the am values mayassume values other than 0 and 1, e.g. 0 to 9 for base-10 and 0 to F forbase-1 6.

Accordingly, it is intended by the appended claims to cover allmodifications which fall within the true spirit and scope of the presentinvention.

1. A method comprising: transmitting a first discrete cosine transform(DCT)-encoded signal which uses at most t coefficient bits to representeach of a plurality of DCT coefficients; and transmitting a secondDCT-encoded signal which uses at most u coefficient bits, wherein u isless than t, to represent each of the plurality of DCT coefficients byremoving at least one lesser-significant bit from each of the DCTcoefficients having t coefficient bits; wherein a single compressionengine transmits the first and second DCT-encoded signals.
 2. The methodof claim 1, wherein the single compression engine transmits the firstand second DCT-encoded signals at substantially the same time.
 3. Themethod of claim 1, further comprising: determining the plurality of DCTcoefficients based on a discrete cosine transform of a plurality ofblocks of data.
 4. The method of claim 1, further comprising:transmitting a third DCT-encoded signal which uses at most v coefficientbits to represent each of the plurality of DCT coefficients by removingat least two lesser-significant bits from each of the DCT coefficientshaving t coefficient bits; wherein v is less than u.
 5. The method ofclaim 1 wherein the first DCT-encoded signal uses t coefficient bits torepresent each of the plurality of DCT coefficients, and wherein thesecond DCT-encoded signal uses u coefficient bits to represent each ofthe plurality of DCT coefficients.
 6. The method of claim 1 wherein thefirst DCT-encoded signal is transmitted via a first data communicationlink having a first bandwidth, wherein the second DCT-encoded signal istransmitted via a second data communication link having a secondbandwidth, and wherein the first bandwidth is greater than the secondbandwidth.
 7. The method of claim 1 wherein the first DCT-encoded signalhas a first data rate, wherein the second DCT-encoded signal has asecond data rate, and wherein the first data rate is greater than thesecond data rate.
 8. The method of claim 1 wherein the first DCT-encodedsignal and the second DCT-encoded signal are substantially synchronized.9. A computer-usable medium comprising a set of instructions to direct aprocessor to perform acts of: transmitting a first discrete cosinetransform (DCT)-encoded signal which uses at most t coefficient bits torepresent each of a plurality of DCT coefficients; and transmitting asecond DCT-encoded signal which uses at most u coefficient bits, whereinu is less than t, to represent each of the plurality DCT coefficients byremoving at least one lesser-significant bit from each of the DCTcoefficients having t coefficient bits; wherein a single compressionengine provides the first and second DCT-encoded signals.
 10. Thecomputer-usable medium of claim 9, wherein the single compression enginetransmits the first and second DCT-encoded signals at substantially thesame time.
 11. The computer-usable medium of claim 9, further comprisinga set of instructions to direct a processor to perform the acts of:determining the plurality of DCT coefficients based on a discrete cosinetransform of a plurality of blocks of data.
 12. The computer-usablemedium of claim 9, wherein the first DCT-encoded signal uses tcoefficient bits to represent each of the plurality of DCT coefficients,and wherein the second DCT-encoded signal uses u coefficient bits torepresent each of the plurality of DCT coefficients.
 13. Thecomputer-usable medium of claim 9 wherein the first DCT-encoded signalis transmitted via a first data communication link having a firstbandwidth, wherein the second DCT-encoded signal is transmitted via asecond data communication link having a second bandwidth, and whereinthe first bandwidth is greater than the second bandwidth.
 14. Thecomputer-usable medium of claim 9 wherein the first DCT-encoded signalhas a first data rate, wherein the second DCT-encoded signal has asecond data rate, and wherein the first data rate is greater than thesecond data rate.
 15. The computer-usable medium of claim 9 wherein thefirst DCT-encoded signal and the second DCT-encoded signal aresubstantially synchronized.
 16. A system comprising: a singlecompression engine to transmit a first discrete cosine transform(DCT)-encoded signal which uses at most t coefficient bits to representeach of a plurality of DCT coefficients and to transmit a secondDCT-encoded signal which uses at most u coefficient bits, wherein u isless than t, to represent each of the plurality of DCT coefficients byremoving at least one lesser-significant bit from each of the DCTcoefficients having t coefficient bits.
 17. The system of claim 16,wherein the single compression engine is operative to transmit the firstand second DCT-encoded signals at substantially the same time.
 18. Thesystem of claim 16, wherein the single compression engine is furtheroperative to determine the plurality of DCT coefficients based on adiscrete cosine transform of a plurality of blocks of data.
 19. Thesystem of claim 16, wherein the first DCT-encoded signal uses tcoefficient bits to represent each of the plurality of DCT coefficients,and wherein the second DCT-encoded signal uses u coefficient bits torepresent each of the plurality of DCT coefficients.
 20. The system ofclaim 16 wherein the first DCT-encoded signal is transmitted via a firstdata communication link having a first bandwidth, wherein the secondDCT-encoded signal is transmitted via a second data communication linkhaving a second bandwidth, and wherein the first bandwidth is greaterthan the second bandwidth.
 21. The system of claim 16 wherein the firstDCT-encoded signal has a first data rate, wherein the second DCT-encodedsignal has a second data rate, and wherein the first data rate isgreater than the second data rate.
 22. The system of claim 16 whereinthe first DCT-encoded signal and the second DCT-encoded signal aresubstantially synchronized.